Mellor, on loan from Preston North End, came off the bench to blast home the Owls' equaliser at the Weston Homes Community Stadium to rescue a point against Colchester United.
Irvine said: "I feel as though he is still finding his feet and I know that he can make an impact coming off the bench.
"But I don't see him just as that – he's not going to just be a sub for us, that's for sure."
Ian Henderson went close for Colchester early on but Wednesday warmed to the task towards the end of an even first half.
Clinton Morrison hooked over from a difficult angle and just before half-time, the experienced striker was only denied an opener by a combination of Paul Reid's challenge and the bottom of the post.
But the Owls fell behind five minutes into the second half when Andrew Bond curled a fine shot past Nicky Weaver. Irvine used all three of his substitutions and changed tactics, in a bid to secure an equaliser.
The changes proved effective and the Owls secured a point when Mellor pounced.
Colchester United: Cousins; Tierney, Okuonghae, Reid, Tierney; Perkins, Izzet, Bond; Wordsworth, Henderson (Vincent, 80), Mooney (Odejayi, 88). Unused substitutes: Baldwin, White, James, Hackney, Pentney.
Sheffield Wednesday: Weaver; Spurr (Jones, 83), Purse, Beevers, Buxton; Teale (Mellor, 59), O'Connor (Potter, 67), Coke, Sedgwick; Tudgay, Morrison. Unused substitutes: Heffernan, Palmer, Otsemobor, O'Donnell.
Referee: D Phillips (West Sussex).
Man of the match: Darren Purse.